Ahead of their encounter at the Etihad on Monday, Koeman - City manager Pep Guardiola's former Barcelona teammate - says that their offensive style of play can leave them light at the back.
"The way he likes to play is the most difficult way," Koeman told reporters. "It's really a pleasure to see his teams playing football.
"They play with a lot of offensive players in the teams. The Premier League is strong physically on set plays and they try to play with a high defence.
"There is always space behind, we know that, we remember that. When we played City at home we punished them for that, but I still think, the business they did in the summer, they are stronger than last year."
City are unbeaten in their last seven home games against Everton, including six Premier League meetings.